As recent technology continues to rapidly increase performance, operation and compactness levels of electronic devices, there is an increasing demand to further reduce sizes and weights of electronic parts for use in electronic devices. This also demands to further improve properties, e.g., heat resistance, mechanical strength and electrical properties of electronic part materials. For example, high levels of density, operation and performance are required for methods of packaging semiconductor devices and for wiring boards mounting such semiconductor devices.
A multilayer printed circuit board generally includes plural layers of insulating substrates. For these interlayer insulating substrates, thermosetting resin prepregs including thermosetting resins impregnated in a glass cloth and films composed of thermosetting resins or photosetting resins have been conventionally used, for example. It is to be desired that the multilayer printed circuit board has a considerably narrow interlayer spacing to increase its density as well as reduce its thickness. This raises a need for an interlayer insulating substrate either with or without a thin glass cloth. Examples of materials known to constitute such an interlayer insulating substrate include rubbers (elastomers), thermosetting resins modified with acrylic resins or others, thermoplastic resins incorporating a large amount of inorganic fillers and the like. In Japanese Patent Laying-Open No. 2000-183539, a method is disclosed for manufacturing a multilayer insulating substrate by incorporating an inorganic filler having a specific range of particle diameters into a varnish comprised chiefly of a high-molecular epoxy polymer and a polyfunctional epoxy resin and coating the resultant onto a substrate to form an insulating layer thereon.
However, the multilayer insulating substrate made by the above-specified manufacturing method must incorporate a large amount of inorganic filler in order to insure a sufficient interfacial contact area between the inorganic filler and the high-molecular epoxy polymer or polyfunctional epoxy resin to improve mechanical strength and other physical properties. This in some cases adds to manufacturing process steps or other inconveniences from the processing point of view and in other cases results in the difficulty to reduce an interlayer spacing, which have been problems.
The thin interlayer insulating substrate, either with or without a thin glass cloth, shows insufficient heat resistance and dimensional stability and often causes inconveniences during a manufacturing process because it is brittle and easy to break, which have been problems.
The multilayer printed circuit board is fabricated such as by a buildup method wherein a sequence of forming a circuit on a layer and superimposing another layer on the circuit is repeated to build up a laminated board, or by a single-operation stacking method wherein circuit-formed layers are stacked together in a single operation. Due to the increased number of processes in either fabrication method, the quality of the material used affects largely on the yield. Due also to inclusion of plating, curing and solder reflowing processes, the material needs to have sufficient solvent resistance, water resistance, heat resistance and high-temperature dimensional stability. Specific examples of such requirements include resistance to acids, alkalis and organic solvents; reduction of moisture absorption that affects electrical properties; high-temperature and post-heating dimensional stability that affects high-precision circuit connection between upper and lower layers; heat resistance up to 260° C. that is required for mounting by lead-free soldering; and reduced occurrence of copper migration that affects reliability of connections.
For example, buildup substrates for use in IC packages and multilayer printed substrates may be placed under high temperature conditions due to heat build-up, but they are still required to maintain high reliability under such conditions. However, a dimensional change of the resin when exposed to high temperatures, if large, causes separation thereof from a metal wiring such as circuit-forming copper to result in occurrence of short-circuiting or wire breaking, which has been a problem. The similar problem occurs even in the flexible multilayer substrate noticed recently as a thin sheet substrate, if an adhesive layer bonding flexible single-layer substrates to each other, a polyimide film constituting the flexible substrate and a metal wiring such as circuit-forming copper differ largely from each other by the degree of dimensional change that they undergo when exposed to heat.
Japanese Patent Laying-Open No. 2000-183539 discloses a technique for improving high-temperature properties by using, in combination, an epoxy resin having superior heat resistance and an inorganic compound. Although the property improving effect is slightly observed at temperatures below a glass transition temperature, it is little observed at temperatures above the glass transition temperature. Neither of moisture absorbency and solvent resistance improving effects can be expected.
Loading of an inorganic filler has been conventionally known to reduce a linear expansion coefficient. This technique is however inapplicable to solder reflowing and other high-temperature treatments. Recent attention to environment has led to the use of a lead-free solder. Since the solder reflowing process temperature continues to increase, the mere use of a highly heat-resistance resin results in the occurrence of inconveniences during high-temperature treatments. That is, the increased linear expansion coefficient of the resin at temperatures above the glass transition temperature causes such inconveniences.
The recent progress of optical communication technology demands an inexpensive way of connecting optical communication devices. Under such circumstances, polymeric optical communication materials have attracted attention. However, the use of conventional polymers as the optical communication materials creates various problems.
The polymeric optical communication material need to be low in loss, superior in heat resistance, low in linear expansion coefficient and low in moisture permeability. Also, they must be readily controllable in refractive index.
By “low in loss”, it is meant that the polymeric material has substantially no absorption band in the wavelength range for use in the optical communication and is thus low in propagation loss.
Japanese Patent Laying-Open No. 2001-183539 describes that conventional polymeric materials exhibit approximately ten times the thermal expansion coefficient of semiconductor or metal material. It also describes that polymeric optical communication material, when formed on a silicon or other substrate having a low thermal expansion coefficient, produces a stress which causes unfavorable results, e.g., polarization dependence of the optical communication material, warpage of the optical communication material and substrate, or separation of the polymeric optical communication material from the substrate.
WO98/45741 describes a problem of a difference in thermal expansion coefficient between an optical fiber (quartz glass) and a resin case that causes the optical fiber to project from a jacket or crack by stress concentration.
Japanese Patent Laying-Open No. Hei 9-152522 describes the case where an optical waveguide substrate is adhesively joined to an optical fiber. A difference in thermal expansion between the optical waveguide substrate and a connector part, if large, is described to cause positional shift during thermal expansion to result in the failure to achieve stable connection to an optical waveguide.
Concerning the moisture permeability, WO 98/45741 describes that a water vapor, if permitted to penetrate into an interior of a hollow case, condenses on a surface of an optical element or fiber to a liquid which problematically causes corrosion of the optical element or promotes growth of cracks leading to breaking of the optical fiber. It also describes that these factors and thermal expansion together lowers reliability of optical communication parts made of polymeric material. Also, the increased moisture absorbency increases the occurrence of light absorption based on an O—H bond of a moisture. This also addresses a need for a material which is low in moisture absorbency.
In order to introduce optical communication to terminal equipments, optical signals must be converted to or from electric signals. In such a case, polymeric optical communication material is used in a printed circuit board or in its vicinities. It is then required that the polymeric optical communication material should show resistance to process temperature during manufacture of a printed substrate as well as to heat radiated from an electric circuit while in use Hitachi Technical Report No. 37 (July, 2001), at pages 7-16, describes solder heat resistance as a prescribed property.
As described above, the optical communication material is expected to have such properties as transparency, heat resistance, low linear expansion coefficient and low moisture absorbency.
Japanese Patent Registration No. 2843314 describes that fluorinated polyimide having a rigid and straight skeleton exhibits a low coefficient of linear thermal expansion.
Japanese Patent Laying-Open No. 2001-108854 discloses a polymeric optical waveguide comprised of a core layer, a clad layer surrounding the core layer and a second clad layer located outside the clad layer and having a lower thermal expansion coefficient than the clad layer. This reference describes that a difference in thermal expansion coefficient between the polymeric optical waveguide and an electric or optical element can be reduced by using different polymers for the clad lay and second clad layer so that the second clad layer has a relatively lower thermal expansion coefficient relative to the clad layer.
Japanese Patent Laying-Open No. 2001-183539 describes an optical communication medium comprised of an insulation film and a substrate and sealed at its ends with a resin which joins them together. This construction is described to prevent the insulating film and substrate from separating from each other at the ends of the medium where stress concentration is likely to occur.
Japanese Patent Laying-Open No. 2001-4850 also describes that the use of a polyimide film having a specific structure for an optical wavelength resin effectively lowers a thermal expansion coefficient.
However, the fluorinated polyimide described in Japanese Patent Registration No. 2843314 is not suitable for use as a clad layer material of the optical communication device because it exhibits lower transparency compared to the other types of polyimides and has a high refractive index of 1.647.
It is suggested in Japanese Patent Laying-Open No. 2001-108854 that an optical wavelength resin containing the particles described therein may satisfy both the low linear expansion coefficient and required transparency. However, the particles must be added in a large amount to actually achieve reduction of linear expansion coefficient. Addition of such a large amount of the particles makes it difficult to achieve sufficient transparency, problematically provides a brittle and weak resin composition, and increases hydrophilicity and moisture absorbency.
The constitution described in Japanese Patent Laying-Open No. 2001-183539 adds to process steps. This inevitably pushes a cost up.
The specifically structured polyimide film described in Japanese Patent Laying-Open No. 2001-4850, when used as an optical wavelength resin, has been found to be difficult to achieve reduction in moisture absorbency, although possible to achieve reduction in thermal expansion coefficient. As a consequence, the cost associated with its use becomes high.
Therefore, it has been difficult to realize an optical circuit forming material which exhibits superior transparency, particularly high transparency, superior heat resistance, low coefficient of linear expansion and low moisture absorbency.
Japanese Patent Laying-Open No. 2002-220513 describes a resin sheet obtained by impregnating a resin containing an inorganic filler into a glass sheet. The obtained resin sheet is described to exhibit a low linear expansion coefficient. However, the actually resulting linear expansion coefficient was not much more than that expected from the use of the glass sheet. Also, an inorganic filler is loaded in the amount of 7% by weight, at maximum, in Examples. The specification provides insufficient description as to a method of dispersing the inorganic filler. It is accordingly hard to find that the inorganic filler is dispersed very finely in the resin.